What could multiple general-purpose processors, of different types, be put to use for? A real-world example of such a system would be the Sega Dreamcast, which has an SH-4 main processor and an Arm7 processor tasked with running the sound chip.

Dist-CC sounds like a good cannidate for the secondary processors. Any other ideas of things that would be good for the secondary processor(s)? Any ideas of how to practically run applications (obviously not C based, but under an interpreter) on all processors(perferably automatically figure out which proc is least taxed at launch)?

This would be pretty amazingly difficult - I can now almost guarantee you that you will not be able to do this with a general-purpose Linux distribution, since it expects the kernel to run on one architecture all the time.

You would have to write an application for the processor(s) to be able to divert processing from the kernel - there is no way the kernel could be made to execute code on the other CPU's.